1. Field of the Invention
The present invention generally relates to a field of disk drives, particularly relates to a data reproducing technique to which a turbo coding/decoding method is applied.
2. Description of the Related Art
A signal processing circuit designated as a read channel, in which a data signal read out from a disk medium (hereinafter simply referred to as disk) with a head is processed to reproduce original data, is generally used in the field of the disk drive typified by a hard disk drive. Usually the signal processing circuit includes a custom LSI and the signal processing circuit containing a write channel processing a write data signal recorded on the disk is also designated as read/write channel or data channel.
The data decoding method (data reproducing method) designated as the so-called PRML (Partial Response Maximum Likelihood), in which a partial response (PR) method and a Viterbi decoding method are combined, is adopted in the current read/write channel.
In contrast to the PRML method, recently another data decoding method in which the PR method and a turbo coding/iterative decoding method are combined receives attention in the field of the disk drive (see Zining Wu, “Coding and iterative detection for magnetic recording channels” Kluwer Academic Publishers, pp. 21–43, 2000).
By the way, the head having a structure in which a read head element and a write head element are separately mounted on a slider is used in the disk drive. A GMR (Giant Magnetic Resistance) element is usually used as the read head element. The GMR element generates a phenomenon called “thermal asperity” in read operation.
In the disk drive, it is confirmed that the data signal read out from the head includes the so-called burst noise in the read operation due to a factor such as the thermal asperity. Because there is a problem in which the burst noise results in disability of error correction and the like, various kinds of measures have been proposed (see Jpn. Pat. Appln. KOKAI Publication No. 2002-164946).
The data decoding method in which the PR method and the turbo coding/iterative decoding method are combined has a configuration in which plural codes are connected through an interleaver. Sometimes this causes the error to be widely spread by the interleaver, in the case that the data signal includes the burst noise as described above. Accordingly, in the case that the error correction is carried out to the decoded data, there is the problem in which the error correction becomes disabled.